In a conventional wireless communication system, the baseband signal at the receiver is converted from an analog format into a digital format so that the useful information in the signal can be recovered via a sequence of digital processes. Typically, an analog-to-digital converter (ADC) is used to achieve this conversion. In general, the more output bits the ADC has, the larger the dynamic range of the input signal the ADC can support. However, this results in a more expensive ADC, as well as higher costs for some of the other receiver components. Given the number of output bits, if the power of the input signal is too large, the output of the ADC may be saturated. On the other hand, if the power of the input signal is too small, the output of the ADC may be severely quantized. For both of these scenarios, the information expected to be recovered at the receiver may be degraded or lost.
A common approach for solving this problem is to apply a dynamically adjustable gain amplifier in front of the ADC so that the input signal of the ADC can be maintained within desired limits. Typically, the adjustable gain is controlled using an AGC circuit.
It is well known in the art that power varies significantly between adjacent timeslots in a TDD frame and between the same time slot in adjacent frames due to variable data rates or a variable number of active users in a timeslot. In order to determine the correct gain level for a given timeslot, the AGC estimates the symbol power of the first N symbols in the timeslot as they are received. During this estimation process, the symbols may be lost for data estimation due to imperfect gain control during this time. Also, depending on the initial accuracy of the gain estimate, this estimation procedure may take a long time; accuracy in this case is the difference between the gain applied at the start of the timeslot and the final “correct” gain as determined by the AGC circuit.
A typical TDD frame generally comprises fifteen timeslots. Each of the timeslots includes two data bursts that are separated by a midamble, followed by a guard period at the end of the timeslot. The data bursts transmit the desired data, and the midamble is used to perform channel estimation.
It would be desirable to have a system and method which avoids the accuracy and data loss problems of current AGC methods.